1. Field of the Invention
The present invention relates to a system for detecting and correcting a position difference of a discoid object and a method thereof, and more particularly to a system for detecting and correcting a position difference of a discoid semiconductor wafer from a predetermined position and a method thereof. Furthermore, the present invention relates to a technique for moving a wafer to a position which is accurately determined, and placing the wafer in that position also when a position difference of a wafer on a conveying robot occurs.
2. Description of the Related Art
When a semiconductor integrated circuit is manufactured, various processes are performed in the state of a wafer. Each of the processes for the wafer is normally performed in a chamber for each of the processes. A robot moves the wafer between chambers. That is, when a process is completed in one chamber, a wafer-moving robot takes a wafer out of the chamber, and moves it to another chamber. At this time, the wafer must be accurately placed at a predetermined position in the chamber.
For example, an R-.theta. robot is well known as a wafer-moving robot. Such a robot has an arm for holding a wafer and can accurately control a length and an angle of the arm. Accordingly, if a wafer is placed at a predetermined position on a hand at the end of an arm of the wafer-moving robot, the wafer is accurately moved to a predetermined position in a target chamber. In other words, if the wafer is placed at a position different from the predetermined position on the hand of the wafer-moving robot, the wafer is not moved to the predetermined position in the target chamber. Therefore, techniques for detecting the position of a wafer on a hand of a wafer-moving robot (or a position difference), and correcting the position difference if it exists have been developed.
As one technique for detecting the position of a wafer, the prior art (U.S. Ser. No. 07/975,197 whose application was filed on Nov. 12, 1992) is known. The technique disclosed in that application is as follows.
A plurality of pairs respectively composed of a light emitter and a light receiver are arranged along a line across an arced path on which a wafer is moved between chambers. If the wafer blocks the light output from each light emitter when the robot hand carrying the wafer is rotated in a direction .theta., a plurality of coordinate points at the leading and trailing edges of that wafer can be determined from the outputs of the light receivers. Two arbitrary coordinate points are selected among the plurality of coordinate points, and a vertical bisector of the segment connecting these two points is drawn. Similarly, another vertical bisector of a segment connecting another two coordinate points is drawn. The intersecting point of these vertical bisectors is defined to be the central position of the wafer.
Japanese laid-open patent publication (TOKKAIHEI) No. 4-295704 is known as another technique for detecting the position of a wafer. With this technique, a linear image sensor which is longer than the diameter of a wafer is arranged, and the wafer is made to pass under the linear image sensor. When the wafer passes through the linear image sensor, the length of a chord is sequentially detected using the linear sensor. Since the longest chord among detected data is the diameter of the wafer, the middle point of the longest chord is detected and that point is defined to be the central position of the wafer.
Some other techniques for detecting the central position of a wafer have been proposed in addition to the above described conventional techniques. Most of them obtain a difference between the central point of a rotation and that of a wafer by rotating the wafer and successively measuring the locus of wafer edges.
The above described implementations according to the conventional techniques respectively have the following problems. One disadvantage of the implementation according to Ser. No. 07/975,197 is that coordinate data cannot be obtained unless a wafer is moved relatively to a sensor.
Generally, the response time of an optical sensor (the response speed of a photoelectric element) is several-hundred microseconds to several milliseconds. Assuming that the response time is 1 millisecond, and the moving speed of the wafer is 500 millimeters per second, the measurement error of 0.5 millimeters arises. To reduce this error, the moving speed of the wafer must be reduced, which leads to the deterioration of the throughput speed of a device.
Additionally, the response time of the optical sensor when the sensor is switched from ON to OFF (when a light receiving element changes from the state in which it receives light to the state in which it does not receive light) is different from the response time when the sensor is switched from OFF to ON (when the light receiving element changes from the state in which it does not receive light to the state in which it receives light). Therefore, the difference between the response times is difficult to be corrected if data detected at the leading and trailing edges are mixed.
Furthermore, since the diameter of a spot of a photoelectric sensor for detecting the blocking and passing of light is normally in the order of 1 millimeter, it is difficult to strictly define that the sensor determines the blocking of light at what percentage of the spot is blocked.
As described above, the implementation according to Ser. No. 07/975,197 is suitable for measuring a relative amount of a move, but it is unsuitable for measuring a position. Additionally, in the implementation according to Japanese laid-open patent publication (TOKKAIHEI) No. 4-295704, a wafer must be moved toward a sensor in a similar manner as in the implementation according to Ser. No. 07/975,197. Accordingly, the move speed of the wafer must be slowed down in order to detect the moment that the length of a chord is the longest with high accuracy. As a result, the throughput speed deteriorates.
For the implementation in which a wafer is rotated, its disadvantage is also pointed out as in the application of Ser. No. 07/975,197, the scale of the detection device is large, and a considerable amount of time is required to rotate and measure the wafer, so that the throughput of the entire device deteriorates.